The present invention relates to a particulate magnetic recording medium of high recording density, and more particularly, to a magnetic recording medium for high-density recording having a magnetic layer and an essentially nonmagnetic underlayer, with the magnetic layer comprising a ferromagnetic metal powder or a hexagonal ferrite powder.
As minicomputers, personal computers, work stations, and other office computers have become widespread in recent years, there has been significant research into magnetic tapes (so-called xe2x80x9cbackup tapesxe2x80x9d) for recording computer data as an external memory medium. As the magnetic tapes employed in these applications have been put to practical use, and particularly as computers have grown smaller and information processing capacity has increased, there has been strong demand for increased recording capacity to achieve high capacity and reduction in size.
In the past, magnetic recording media in which a magnetic layer comprising iron oxide, Co-modified iron oxide, CrO2, a ferromagnetic metal powder, or a hexagonal ferrite powder dispersed in a binder is coated on a nonmagnetic support have been widely employed. Of these, ferromagnetic metal powders and hexagonal ferrite powders are known to have good high-density recording characteristics.
To improve the characteristics of a disk magnetic recording medium, the use of vinyl chloride resin having an acid group, epoxy group, and hydroxyl group is proposed in Japanese Unexamined Patent Publication (KOKAI) Showa No. 64-84418; the use of a metal powder with an Hc equal to or higher than 80 kA/m (1,000 Oe) and a specific surface area ranging from 25 to 70 m2/g is proposed in Japanese Examined Patent Publication (KOKOKU) Heisei No. 3-12374; and the incorporation of an abrasive to stabilize the magnetization level and specific surface area of the magnetic material is proposed in Japanese Examined Patent Publication (KOKOKU) Heisei No. 6-28106.
To improve the durability of the magnetic recording medium, the incorporation of a nonmagnetic powder with a Mohs"" hardness equal to or higher than 6 and a higher fatty acid ester is proposed in Japanese Unexamined Patent Publication (KOKAI) Showa No. 54-124716; the setting of the volume of pores in a lubricant and the setting of the surface roughness to the range of 0.005 to 0.025 xcexcm is proposed in Japanese Examined Patent Publication (KOKOKU) Heisei No. 7-89407; the use of a low-melting-point fatty acid ester and an abrasive with a particle diameter of from one-fourth to three-fourths the thickness of the magnetic layer is proposed in Japanese Examined Patent Publication (KOKOKU) Heisei No. 7-36216; and the use of chromium oxide and a metal magnetic material comprising Al is proposed in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 3-203018.
As the configuration of a magnetic recording medium having a nonmagnetic lower layer and intermediate layer, a configuration having an electrically conductive layer and a magnetic layer having a metal powder is proposed in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 3-120613; a configuration having a magnetic layer equal to or lower than 1 xcexcm and a nonmagnetic layer is proposed in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 6-290446; a configuration comprising an intermediate layer of carbon and a magnetic layer containing a lubricant is proposed in Japanese Unexamined Patent Publication (KOKAI) Showa No. 62-159337; and a configuration having a nonmagnetic layer with carbon of specified size is proposed in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 5-290358.
As minicomputers, personal computers, and other office computers have become widespread in recent years, there has been significant research into magnetic tapes (so-called xe2x80x9cbackup tapesxe2x80x9d) for recording computer data as an external memory medium. As the magnetic tapes employed in these applications have been put to practical use, and particularly as computers have grown smaller and information processing capacity has increased, there has been strong demand for increased recording capacity to achieve high capacity and size reduction. Further, demand has increased for further improvement in magnetic tapes for use under a wide variety of environmental conditions (particularly conditions of sharp fluctuation in temperature and humidity) due to the varied environment in which these magnetic tapes are employed, as well as for reliable data storage, stable recording of data during high-frequency travel with repeated use at high speeds, and reliable performance in reading and the like.
Conventionally, the magnetic tapes employed in digital signal recording systems are determined by the system; there are known magnetic tapes corresponding to DLT-type, 3480, 3490, 3590, QIC, D8-type, and DDS-type systems. Irrespective of the system in which employed, a magnetic layer comprising a ferromagnetic powder, binder, and abrasive of single-layer structure with a relatively thick film thickness ranging from 2.0 to 3.0 xcexcm is provided on one side of a nonmagnetic support, and a backcoat layer for preventing tangled winding and ensuring good running durability is provided on the other side of the magnetic tape. However, in such magnetic layers of relatively thick single-layer structure, there is generally a problem in the form of loss due to thickness resulting in decreased output.
Thinning of the magnetic layer is known to afford improvement by reducing the drop in reproduction output caused by the thickness loss of the magnetic layer. For example, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 5-182178 discloses a magnetic recording medium in which a lower nonmagnetic layer comprising an inorganic powder dispersed in resin is provided on a nonmagnetic support and an upper magnetic layer equal to or less than 1.0 xcexcm in thickness comprising a ferromagnetic powder dispersed in binder is provided over said nonmagnetic layer while said nonmagnetic layer is wet.
However, with the rapidly increasing capacity and high densification of magnetic recording media, it is difficult to achieve satisfactory characteristics with such technology. It has also been difficult to achieve durability simultaneously.
Accordingly, the object of the present invention is to provide a magnetic recording medium with improved electromagnetic characteristics, particularly improved high-density recording characteristics, combined with good durability, particularly a substantially improved C/N ratio in the high-density recording region.
The present inventors conducted extensive research into obtaining a magnetic recording medium with good electromagnetic characteristics and durability, and particularly, a markedly improved C/N ratio in the high-density recording region, resulting in the discovery that the good high-density recording characteristics and good durability targeted by the present invention were achieved by the medium set forth below; the present invention was devised on that basis.
That is, the present invention relates to a magnetic recording medium comprising a nonmagnetic lower layer and a magnetic layer provided in this order on a support wherein said magnetic layer comprises a ferromagnetic metal powder or a ferromagnetic hexagonal ferrite powder and a binder and has a coercive force equal to or higher than 143 kA/m, wherein said magnetic recording medium is a medium for recording signals having a surface recording density ranging from 0.2 to 2 Gbit/inch2, said support has a thickness equal to or less than 5.5 xcexcm and a Young""s modulus in the MD direction equal to or higher than 11,000 Mpa, and said support has on the reverse surface from said magnetic layer a backcoat layer, said backcoat layer having not fewer than 200 and not more than 1,000 protrusions having a height equal to or higher than 50 nm per 10,000 xcexcm2.
The above-stated object of the present invention is achieved by making it possible in the magnetic recording medium of the present invention to maintain, without loss due to the effect of the protrusions present in the above-described backcoat layer, the good high-density recording characteristics obtained by the use of a magnetic layer in which a ferromagnetic metal powder or a ferromagnetic hexagonal ferrite powder is dispersed in a binder; by the use of a backcoat layer having not fewer than 200 and not more than 1,000 protrusions equal to or higher than 50 nm in height per 10,000 xcexcm2 to yield good running durability; and by the use of a support with a thickness equal to or less than 5.5 xcexcm and a Young""s modulus in the MD direction equal to or higher than 11,000 Mpa. Based on the present invention, both good high-density characteristics and good durability are combined, yielding a magnetic recording medium with a markedly improved C/N ratio in the high-density area and yielding computer tapes.
In the present invention, it is further desirable for the dry thickness of the magnetic layer to be in a range of from 0.05 to 0.25 xcexcm, for "PHgr"m to be in a range of from 1.0xc3x9710xe2x88x925 to 1.3xc3x9710xe2x88x925T (from 8.0xc3x9710xe2x88x923 to 1.0xc3x9710xe2x88x923 emu/cm2), and for the lower layer and/or the magnetic layer to comprise at least a fatty acid and a fatty acid ester to permit obtaining a magnetic recording medium with a markedly improved C/N ratio in the high-density recording area having both good high-density characteristics and good durability, previously impossible with prior art.
In the present invention, it is further desirable for the magnetic recording medium to record signals at a surface recording density ranging from 0.2 to 2 Gbit/inch2, and for the lower layer to comprise an inorganic powder with a Mohs"" hardness equal to or higher than 4 to improve durability during repeated run.
In the present specification, the term xe2x80x9cnonmagnetic lower layerxe2x80x9d includes a lower layer having magnetism of a degree not contributing to recording; this layer may be referred to hereinafter as simply the lower layer or nonmagnetic layer.
"PHgr"m refers to the magnitude of the magnetic moment capable of being directly measured at Hm 796 kA/m (10 kOe) with a vibrating sample magnetometer (VSM: from Toei Kogyo Co.,Ltd.) on one side of a magnetic layer of unit area; this is equal to the magnetic flux density Bm (unit: T (tesla)) obtained by VSM multiplied by the thickness (m). Thus, the unit of "PHgr"m is denoted as Txc2x7m.
The term xe2x80x9clinear recording densityxe2x80x9d is the number of bits of signal recorded per inch in the recording direction.
The linear recording density, track density, and surface recording density are values determined by the system. That is, in improving the surface recording density in the present invention, the thickness of the magnetic layer and magnetic layer Hc are used to improve the linear recording density, and optimization of "PHgr"m is used to improve track density.